This is a 0.3X Barlow (auxiliary objective) lens for stereo microscopes with 48mm threading. It reduces magnification by 0.7X, and increases the working distance to a maximum of 287mm (11.3in). The mounting thread is 1-7/8" (48mm), designed for AmScope SM and SW series stereo microscopes. Due to the large working-distance, this Barlow lens is only intended for microscopes which support appropriate height-adjustment.